As a spark plug used as ignition means in an internal combustion engine, such as an engine of an automobile, there is a spark plug in which a center electrode and a ground electrode are made to axially face each other to form a spark discharge gap. Such a spark plug generates discharge at the spark discharge gap and ignites an air-fuel mixture in a combustion chamber by the discharge.
In the combustion chamber, airflow of the air-fuel mixture, for example, such as swirl flow or tumble flow, is formed and the airflow moderately flows also at the spark discharge gap to thereby ensure ignitability.
However, depending on a mounting posture of the spark plug to the internal combustion engine, part of the ground electrode joined to a tip end part of a housing may be arranged on an upstream side of the spark discharge gap in the airflow. In this case, the airflow in the combustion chamber is blocked by the ground electrode, and the airflow near the spark discharge gap may stagnate. As a result, the ignitability of the spark plug may degrade. That is, the ignitability of the spark plug may vary depending on the mounting posture to the internal combustion engine. Particularly in recent years, lean combustion internal combustion engines are used widely. However, in such an internal combustion engine, depending on the mounting posture of the spark plug, combustion stability may be degraded.
Unless special measures are taken, it is difficult to control the mounting posture of the spark plug to the internal combustion engine, that is, the position of the ground electrode in a circumferential direction. This is because the mounting posture varies due to the formation state of a mounting screw in the housing or the tightening degree of the spark plug at the time of mounting to the internal combustion engine. Special measures may be considered such that limiting the relation between the mounting screw and the joining position of the ground electrode in the circumferential direction of the spark plug to a specific positional relation, and also limiting the orientation of a female screw on an engine head side to a predetermined direction in the circumferential direction. However, in this case, manufacturing man-hours and manufacturing cost of the spark plug and the engine head may be increased.
Thus, to suppress blocking of airflow by the ground electrode, a configuration in which drilling processing is performed in the ground electrode, and a configuration in which the ground electrode is joined to the housing by a plurality of thin plate members are disclosed (PTL 1).